(a) Field of the Invention
The present invention relates to an easily openable container which is suited for food packaging, especially, for retort sterilization and to the method of producing the easily openable container.
(b) Description of the Related Art
The most conventional containers for packaging food include a lid member and a container body having a flange, where the container body is melt sealed with the lid member at the flange. In a conventional method of imparting both easy openability and sealability to the containers, a multilayer container body having an interlaminar strength smaller than the peel strength between the flange and the lid member is used and a notch is made in the sealing layer of the multilayer container body between the sealed area and the inner edge of the flange so that the opening of the container is performed by the peeling of the sealing layer between the outer edge and the flange to the notch. The method caused a demand for the increase of the stiffness of the flange, for example, by providing a skirt to the flange of a tray-shaped container.
Opening of the containers including a skirted flange required partial removal of the skirted flange at the portion for beginning opening, which was performed by corner-cutting or punching of the skirted flange. For example, in Japanese Patent Application Kokai Koho (Laid-open) Nos. 2-19280 and 2-98560 disclosed are easily openable containers whose flanges have at the flange a hole for facilitating the beginning of opening from the hole, and the hole is made by punching the flange of a sealed container at the portion for beginning opening or by sealing a container body and a lid each having a hole at the portion for beginning opening. There is, however, a problem that the method needs a punching step which complicates the processing procedures, the stiffness of the flange is decreased and, further, the appearance of the obtained container is deteriorated. In Japanese Patent Application Kokai Koho (Laid-open) No. 63-307072 disclosed is an easily openable container where a circular depression is made in the sealing layer of the container body at the outer periphery of the melt sealed area by pressing the innermost layer through the lid member at the time of melt sealing, thereby facilitating beginning of opening from the circular depression. In Japanese Patent Application Kokai Koho (Laid-open) No. 63-307073 disclosed is an easily openable container where two circular depressions are made in the sealing layer at both the outer and inner periphery of the melt sealed area in the same manner as in the method of Japanese Patent Application Kokai Koho (Laid-open) No. 63-307072. These methods also involve a problem that opening of the easily openable containers needs the breaking of the sealing layer at the circular depressions in the whole circumference of the flange, which causes a problem of a bad appearance at the broken portion of the sealing layer, such as beard-like breaking line. Further, the easily openable container disclosed in Japanese Patent Application Kokai Koho (Laid-open) No. 63-307073 involves another problem that it has a poor strength to internal pressure since there is no space between circular depression at the inner periphery of the melt sealed area and the melt sealed area, and if the sealing is made strongly to increase the strength to internal pressure, openability is decreased.